1. Field of the Invention
The present invention relates to means for applying power to an AC load and is more particularly concerned with the application of power at the crest of an oscillating voltage waveform to thereby prevent current surges during activation of the AC load.
2. Description of the Prior Art
With certain types of AC loads and particularly with those requiring a substantial amount of power for initial activation such as the primary winding of a transformer used to drive a magnetron power supply in a microwave oven, a means must be employed to limit current surges during start-up; otherwise, fuses and circuit breakers used to prevent excessive current flow would be continuously thrown open as a result of excessive current being drawn by the load or in the absence of a fuse, circuit breaker, or other means for preventing excessive current flow, wiring and/or the AC load itself may be damaged. Current surges are produced by activation of the AC load at random phases of the alternating reference signal. One method of preventing such surges is by activation of the load at the crest or peak voltage of the alternating signal. Typically, rather than attempt to have the load activate or draw power for activation at the crest of an alternating reference signal, a current limiting device such as a resistance means will be electrically coupled in series with the load to prevent current surges. Accordingly, the larger the value of the resistance means the less current required to provide the necessary power to activate the load.
The present invention provides a means for applying power to an AC load at the voltage crest of an alternating reference signal thereby alleviating the necessity for a current limiting device to prevent current surges during the activation of the load. Employed in the present invention is a light emitting diode which fires or turns-on a semi-conductor switching device at the crest of an oscillating voltage waveform thereby activating the AC load. The invention utilizes two important light emitting properties of light emitting diodes when driven by a voltage source to achieve the objects enumerated hereinafter. These important properties are (1) that the light emitted by the light emitting diode is proportional in intensity to the instantaneous current flowing through the light emitting diode when biased in a forward direction and (2) that the current flowing through the light emitting diode depends exponentially on the applied voltage in a manner characteristic of junction diodes in general; namely, very little current flows through the light emitting diode when the applied voltage is below an established threshold voltage and once the applied voltage has reached the threshold voltage the current and therefore luminous intensity increases very rapidly (i.e. exponentially) as the applied voltage continues to increase.